Rules of DOJ, did not allow him to charge the president regardless of evidence so he decided not to make accusations because it would be unfair to do so. As he said, "It would be unfair to potentially accuse somebody of a crime when there can be no court resolution..." His job was to determine if there was sufficient evidence to pursuit an indictment before the grand jury. It certainly was not to issue press statements about his opinion of guilt or innocence of the president.
Wait...
Mueller could not, under DOJ rules conclude the President committed a crime?
Mueller, under the pretense of fairness, could not accuse, or potentially accuse, the President of a crime?
Really?